Stacy McNulty: Beech Nuts, Mice and Bears

What follows is a guest essay by Stacy McNulty, Associate Director of SUNY Environmental Science and Forestry’s Adirondack Ecological Center in Newcomb. McNulty and her colleagues recently conducted a study of how the availability of forest mast affects small mammals.

Have you noticed a mouse explosion in your camp or garage this summer? Are black bears making mincemeat of your garbage cans?

This summer, reports of stories of Adirondack bears breaking into in candy stores and making off with campers’ food abound. The dry spring has contributed to the scarcity of food in the woods. Yet there is another reason why we’re sometimes overrun with these animals.

Recently, my colleagues and I completed an in-depth look at population cycles in several mammals using two decades of data from SUNY ESF’s Huntington Wildlife Forest in Newcomb, NY and from state harvest records. We found clear relationships between predators and prey in the Adirondacks and beyond.

A key determinant of animal numbers is food availability. Many plants produce seed periodically; oaks are well-known for a 2 to 5 year cycle of acorn production. American beech trees produce a nut crop about every two years. This cyclical seed source, known as mast, is thought to overwhelm seed-eaters (some seeds escape detection and become plants). Mast cycles may also be an adaptation to the short growing season that makes it difficult to put energy into seeds every year rather than plant growth.

In the Adirondacks, beech mast is important to many species: bears and deer as well as turkeys, grouse, rodents, blue jays and some insects. Bears travel long distances for energy-rich foods in preparation for winter; high-fat beech nuts are sought-after. When beech nuts aren’t available, bears turn to other foods. In fact, the mast pattern is so clear we can predict bear cub production across New England and Canada using central Adirondack beech nut numbers.

There are also patterns in the numbers of American marten and fisher relative to the availability of beech mast. Data indicated Adirondack trappers harvested these two species of the weasel family more often in the year following a beech mast event. This time lag makes sense for two reasons. First, marten and fisher prey on the small mammals that reproduce following high tree seed production. Small mammals reach peak abundance when juvenile martens and fishers are growing, thus, survival is likely higher following a beech mast event. Second, these two furbearers eat beech nuts as well as fruit (especially mountain ash), birds and insects.

To illustrate the most recent cycle: in fall 2011, many trees in the central Adirondacks produced mast, especially beech, sugar maple, red spruce, balsam fir, and Eastern white pine. This heavy seed crop followed by the mild winter created conditions for a huge spike in rodents in 2012. ESF’s Adirondack Ecological Center records show a 20-fold increase in live-caught mice from 2011 to 2012 and the highest mouse population in over ten years (see accompanying chart). Voles, squirrels and other small mammals increased too, and while 2012 data aren’t available yet, predator numbers may have increased over the summer due to this abundance of prey.

This body of research is relevant to readers for several practical reasons, including:

In addition to the role of mice as food for a variety of predators, they impact forest soil, consume a variety of insect pests, and are important in both seed and fungi dispersal.

Mice can also be a health risk. The first line of defense against a mouse home invasion is plugging holes near your foundation. Use of snap traps to remove them is more humane than poisons like D-con. Also, poisoned mice that go outside can be consumed by cats, owls, and hawks and kill these predators unintentionally.

Hunters and trappers may locate game more easily when there are high numbers of young and naive animals (or when food scarcity makes animals more susceptible).

Squirrels and chipmunks also eat songbird eggs and nestlings and provide food for mid-sized carnivores like coyotes and foxes.

For the coming fall and winter, there is little sign of a large seed crop, meaning the cycle will shift to a period of scarcity for many animals. Another recent SUNY ESF study reported Adirondack bear-human conflicts spike in the summer before a beech mast failure – suggesting an environmental cue driving natural food and bear activity patterns. This seems to be the case for 2012.

The patterns in wildlife numbers in the long record for Huntington Wildlife Forest are strong, and these species interactions impact human as well as natural communities. My house is overrun with mice this year, despite my indoor cat’s vigilant efforts. Whether you enjoy the antics of chipmunks or curse their raids on your garden, it’s an opportunity to see if our models correctly predict a famine in 2012 as they did the feast of 2011.

LaMere, C. R., S. A. McNulty and J. E. Hurst. In press. Human-black bear conflicts are related to mast production in the Adirondack Mountains of New York State. Proceedings of the Eastern Black Bear Workshop 2011.

Great information and story, Stacy. Thank you. And just south of the Adirondacks, the mouse explosion means more carriers of the Lyme bacillus spread by the black legged (deer) tick. One of the implications of your story is that in simplified environments in developed landscapes there may be fewer controls on the mouse population, and heightened lyme transmission?

I’ve been waiting for some information on this since I noticed the increase in rodents here and definitely more owls(as a result?) Thank you,Stacy for this good information..how does the acorn production affect things? Our Oaks are loaded this year..not only more but larger acorns..falling faster than the squirrels and turkeys can eat them. I’m in the Tug Hill area.

[…] biologists say the increased encounters are the result of reduced natural food sources this year [read more]. Black bears will take advantage of readily available food sources, including bird feeders and […]

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